Everyone has great suggestions!
If she wants a flower, I would see if she cares if it's just a simple flower.. or if it has to be calla lily. I love the look of the bitty bands, with 3 smaller flowers in a row.. I think they are 1-1.5 inches. I normally see them with roses. To me, it's dainty, plus you don't really pay attention to the kind of flower in the bow.. it's just a fancy headband with pretty flowers. It's something that you could do all black or all red. Simple and classy.
If she is wanting a bow.. you could do what others suggested, the satin ribbon with some crystal embellishments. If she is having a more fancy wedding, you could do an organza base bow.